Constantine Laskaris was born of a noble but not particularly renowned Byzantine family. Virtually nothing is known of him prior to the events of the Fourth Crusade . He found favour after his brother Theodore married into the imperial family, becoming the son-in-law of emperor Alexius III . During the first siege of Constantinople in 1203 he was given command of the best body of troops available and led the Greek defenders on sorties against the entrenched Crusaders. None were successful in their goal of lifting the siege, and finally Constantine was ordered to attack the Burgundians who were on guard at the time. The Greeks issued forth from the city, but were soon driven back to the gates, notwithstanding the stones that the defenders on the walls threw down onto the advancing Crusaders. Constantine himself was captured whilst mounted on his horse by William of Neuilly and probably kept for ransom, which was the usual practice of the times. At some point he was released, as he was soon swept up in the events of the second siege of Constantinople in 1204.

After the Crusaders entered Constantinople on 12 April 1204 and began to sack the city, a large body of citizens as well as what remained of the Varangian Guard gathered together in the church of Hagia Sophia to elect a new emperor, as Alexius V had fled the city. Two nominees presented themselves - Constantine Laskaris and Constantine Doukas (probably the son of John Angelos Doukas, and thus a first cousin to Isaac II and Alexius III ). Both presented their case to be nominated emperor, but the people could not decide between them, as both were young and had proven military skills. Eventually lots were cast and Laskaris was selected by what remained of the army as the next emperor.
Laskaris refused to accept the imperial purple; escorted by the Patriarch of Constantinople, John X , to the Milion , he urged the assembled populace to resist the Latin invaders with all their strength. However, the crowd was unwilling to risk their lives in such a one-sided conflict, and so he turned to the Varangians and asked for their support. Though his pleas to honour fell on deaf ears, they agreed to fight for increased wages, and he marched out to make a final stand against the Latin Crusaders. However, the Varangians betrayed Constantine and fled at the sight of the mail-clad Latin troops. Seeing all was lost, he quickly fled the capital in the early hours of 13 April 1204..

Greek resistance to the Latin conquerors began almost immediately under the leadership of Theodore Laskaris , and he was soon joined by Constantine. They were hard pressed at first, and by early 1205 they had lost the important city of Adramyttium to Henry of Flanders . Theodore was keen to reverse this setback, and so he sent Constantine at the head of a large body of troops towards the city. Henry of Flanders had advance warning of the attack via an Armenian source, and prepared his forces to meet the Greeks. The two armies fought the Battle of Adramyttium on Saturday, 19 March 1205 outside the city walls, and the result was a massive defeat for Constantine Laskaris and the Greeks, with most of the army either perishing or being captured.
Nothing more is heard of Constantine Laskaris after this battle, so it is presumed that he either perished in the defeat, or was captured.

Constantine had six brothers: Manuel Laskaris (died after 1256), Michael Laskaris (died 1261/1271), Georgios Laskaris, Theodore , Alexios Laskaris, who fought with the French against John III Doukas Vatatzes and was imprisoned and blinded, and Isaakios Laskaris.
According to "The Latins in the Levant. A History of Frankish Greece (1204-1566)" (1908) by William Miller, the seven brothers may also have had a sister. Miller identified said sister with the wife of Marco I Sanudo and mother of Angelo Sanudo . He based this theory on his own interpretation of Italian chronicles. The "Dictionnaire historique et Généalogique des grandes familles de Grèce, d'Albanie et de Constantinople" (1983) by Mihail-Dimitri Sturdza rejected the theory based on the silence of Byzantine primary sources.

The primary source for the elevation of Constantine Laskaris is Niketas Choniates an eyewitness who recounted the fall of Constantinople to the Crusaders. However, given Constantine's apparent subordinate role under his brother Theodore in 1205, historians such as Sir Steven Runciman and Donald Queller have argued that it was in fact Theodore and not Constantine who was in Hagia Sophia that fateful day, and it was Theodore who was nominated and thus succeeded Alexius V . This uncertainty, plus the fact that Constantine remained uncrowned means that he is not always counted among Byzantine Emperors. Therefore the convention when it comes to Constantine Laskaris is that he is not usually assigned a numeral. If he is counted as Constantine XI, Constantine XI Palaeologus , the last Emperor, is counted as Constantine XII